A conventional disk driving device applied to a hard disk driving device is shown in FIG. 3. A cup-shaped frame 5 includes a cylindrical bearing holder 5a at the center thereof. A flange-shaped mounting portion 5c is radially outwardly extended from the upper end of a cylindrical wall 5b of the frame. And a stator core 6 is mounted on the outer cylindrical surface of the bearing holder 5a. The stator core 6 is formed by stacking a plurality of core plates made of magnetic substance, and has a plurality of protruded poles which are radially outwardly extended from the center thereof. A drive coil assembly 7 is wound on the protruded poles of the stator core 6.
Upper and lower ball bearings 1 and 1' are fixed in the bearing holder 5a of the frame 5 in such a manner that their outer races are fixed to the inner cylindrical surface of the bearing holder 5a, and their inner races are fixed to the central shaft 2b of a hub 2 on which for instance a magnetic disk is placed. The hub 2 is cylindrical, and its outer peripheral portion is stepped downwardly, thus being formed into a disk mounting portion 2d. The upper surface of the latter 2d includes an annular disk mounting surface 2c which is protruded slightly upwardly. The central portion of the hub 2 is protruded downwardly, thus forming the aforementioned central shaft 2b. The shaft 2b is fixed to the inner races of the upper and lower ball bearings 1 and 1'. That is, the hub 1 is rotatably mounted on the frame 5 through the ball bearings 1 and 1'. The hub 2 is made of a non-magnetic material such as for instance aluminum alloy.
A yoke 3 is fixedly secured to the outer periphery of the lower surface of the disk mounting portion 2d. The yoke 3 includes a cylindrical portion 3a, and a bent portion 3b which is radially inwardly extended from the upper end of the cylindrical portion 3a. A protrusion 2e extended from the lower surface of the disk mounting portion 2d of the hub 2 is caulked over the bent portion 3b, so that the yoke 3 is fixedly secured to the hub 2. An annular drive magnet unit 4 is fixedly mounted on the inner cylindrical surface of the cylindrical portion 3a of the yoke 3 in such a manner that the inner cylindrical surface of the magnet unit 4 is confronted through a predetermined gap with the protruded poles of the stator core 6. Hence, by applying current to the drive coil assembly 7 wound on the protruded poles of the stator core 6, the drive magnet unit 4 is energized, so that the yoke 3 and the hub 2 are rotated.
In the above-described disk driving device, the yoke is secured to the hub by caulking. However, the method of securing the yoke to the hub is not always limited thereto or thereby. For instance, methods as shown in FIGS. 4(a) and 4(b) may be employed. In the method shown in FIG. 4(a), the outer cylindrical surface G of a disk-shaped hub 2, and the lower surface H of the outer peripheral portion of the hub 2 are joined through adhesive to the inner cylindrical surface of the upper end portion of a yoke 3' and the upper end face of the drive magnet unit 4, respectively. The method has been disclosed in Japanese Utility Model Publication No. Hei. 5-91167.
In the method shown in FIG. 4(b), a protrusion 2e is formed on the lower surface of the hub 2, and the peripheral portion I of the lower surface of the hub 2 which surrounds the protrusion 2e and the outer cylindrical surface J of the protrusion 2e are joined through adhesive to the inner periphery of the upper surface of the bent portion 3b of the yoke 3 and the inner cylindrical surface of the head portion 3b, respectively. The method has been disclosed in Japanese Utility Model Publication No. Hei. 6-38060.
In the conventional device shown in FIG. 3, When the yoke 3 is fixedly secured to the hub 2 by caulking, the yoke 3 is positively secured to the hub 2, so that the yoke 3 never comes off the hub 2. However, the hub 2 is unavoidably strained during caulking, so that its disk mounting surface 2c is somewhat deformed. Hence, when the hub 2 is turned with a disk mounted on the annular disk mounting surface 2c thus deformed, the disk is oscillated axially, that is, a so-called "planar oscillation" occurs. In order to eliminate this difficulty, it is necessary to finish the annular disk mounting surface 2c by the lathe processing or the like after the yoke 3 and the drive magnet unit 4 are secured to the hub 2.
On the other hand, in each of the methods shown in FIGS. 4(a) and 4(b), the yoke is bonded to the hub with adhesive. Hence, the methods will not adversely affect the disk mounting surface such as the aforementioned method, that is, by caulking. However, in each of the methods, the bonding surfaces are so small that the yoke is not sufficiently fastened to the hub. Hence, if an excessively great impact is applied to the device, the yoke may come off the hub, thus damaging other components.